In the lithography process for semiconductor devices, such as LSIs and VLSIs, liquid crystal display panels, and the like, a pellicle is mounted on a lithography mask for preventing possible attachment of foreign substance to the mask. A pellicle generally includes: a pellicle frame; a transparent pellicle membrane stretched over the upper end surface of the pellicle frame; a mask adhesive layer provided on the lower end surface of the pellicle frame for sticking the pellicle to a musk; and a tacky layer provided on the inner surface of the pellicle frame.
The tacky layer provided on the inner surface of the pellicle frame not only prevents generation of dust from the pellicle frame but also can hold foreign substance, such as dust suspended in the space inside the pellicle frame. Therefore, the tacky layer is effective for avoiding possible attachment of foreign substance to the mask.
In recent years, reduction in the wavelength of exposure light has advanced with the miniaturization of mask patterns. Examples of short-wavelength light include excimer light, such as KrF excimer laser and ArF excimer laser.
FIG. 5 is a schematic diagram showing an example of a pellicle mounted on a mask. As shown in FIG. 5, exposure light generally passes through a portion other than light-blocking pattern 9 of mask 5 and is incident generally parallel to the normal line of pellicle membrane 1 (refer to a thick solid line arrow and a dotted line arrow in the drawing). Therefore, the exposure light does not directly hit tacky layer 8 on the inner surface of pellicle frame 3. However, a part of the exposure light may be scattered at the end (edge) of light-blocking pattern 9 of the mask, and the scattered light may hit tacky layer 8 on the inner surface of pellicle frame 3 (refer to thin solid line arrows in the drawing). In recent years, the exposure light may be allowed to be incident obliquely to the normal line of pellicle membrane 1 (not shown). Therefore, the influence of the scattered exposure light on the tacky layer has increased.
In the tacky layer provided on the inner surface of a conventional pellicle frame, degradations, such as burning, evaporation, solidification and/or mobilization may occur when short-wavelength light having high energy as described above hits the tacky layer.
Moreover, in the lithography using ArF excimer laser light, when a pellicle is used for a long period of time, solid foreign substance may be deposited (haze may occur) on a mask within a space surrounded by the pellicle frame. A possible mechanism of the deposition of solid foreign substance will now be described. If the scattered exposure light or the like hits the tacky layer provided on the inner surface of the pellicle frame, substances in the tacky layer will decompose, and the resulting outgas components (decomposed gas such as ions and organic gases) will be released to a space surrounded by the pellicle frame. Then, the released outgas components will undergo a chemical reaction caused by ArF excimer laser light resulting in deposition of solid foreign substance. It is therefore required for the tacky layer provided on the inner surface of the pellicle frame not to generate decomposed gas even if the tacky layer is irradiated with scattered light of ArF excimer laser light or the like and thereby to prevent deposition of solid foreign substance.
Therefore, it has been required in the art that the tacky layer provided on the inner surface of the pellicle frame have excellent holding performance (tackiness) that allows for satisfactory fixation of suspended dust and the like to the tacky layer, and excellent light resistance to the short-wavelength light such as excimer light so as not to cause the above-described degradations or deposition of solid foreign substance.
There are also proposed a pellicle having a tacky layer containing, for example, a perfluoropolyether and a viscosity controlling agent on the inner surface of a pellicle frame (PTL 1); a pellicle having a tacky layer containing a fluororubber (PTL 3); a pellicle having a tacky layer containing fluororesin grease (A) and a high-molecular weight fluororesin (B) (PTL 4); and the like. Moreover, in order to increase the light resistance to an ArF excimer laser, there are proposed a pellicle having a tacky layer made of a cured product of a composition containing a perfluoro compound on the inner surface of the pellicle frame (PTL 2); and the like.